Add widget support to SourceColumn

Allow passing a widget class to SourceColumn. This makes
using lambdas to render model column unnecesary and are
mostly removed too.

Signed-off-by: Roberto Rosario <Roberto.Rosario@mayan-edms.com>

mayan/apps/document_states/models.py

@@ -318,6 +318,7 @@ class WorkflowStateAction(models.Model):
 
     def get_class_label(self):
         return self.get_class().label
+    get_class_label.short_description = _('Action type')
 
     def loads(self):
         return json.loads(self.action_data)